1. Technical Field
The present invention relates to a fan motor control unit that suppresses degradation in air volume characteristic and an increase in power loss, despite an increase in load imposed on the fan motor.
2. Description of Related Art
Brushless motors have thus far been employed for use as fan motor, because the rotation speed can be controlled over a wide range. The brushless motor includes, for example as disclosed in Japanese Unexamined Patent Application Publication No. 2006-180608, a plurality of Hall elements to detect the rotational position of the rotor according to signals outputted by the Hall elements.
Energization timing for the stator coil in the brushless motor is set in advance with respect to each of the rotational positions of the rotor. Here, the energization timing refers to the timing at which the stator coil is connected to the power source.
In existing fan motor control units, however, the energization timing for the stator coil is constant. Accordingly, as shown in FIG. 10, the rotation speed (broken lines) of the fan motor drops, the air volume decreases, and the static pressure of the fan increases, as the load on the fan motor becomes higher (solid lines). In other words, the air volume characteristic is degraded as the load on the fan motor is increased.
To be more detailed referring to FIG. 10, the rotation speed of the fan motor is highest in a free-air state in which the load on the fan motor is lightest, and the rotation speed of the fan motor becomes lower as indicated by the broken lines toward a high-load state through a low-load state, until the rotation speed becomes lowest at a highest-load state in which the fan motor is subjected to a heaviest load. Therefore, as indicated by solid lines in FIG. 10, the air volume decreases and the static pressure of the fan increases, with the increase of the load imposed on the fan motor. Here, the highest-load state refers to a fully closed state in which the air inlet is fully closed when the fan is mounted in the casing.
Further, as shown in FIGS. 11A to 11D, the waveform of the current flowing through the stator coil becomes more irregular as the fan motor shifts toward the higher load state, until the fluctuation range of the current expands to the extent that needle-shaped waveforms appear each time the stator coil is switched. Thus, the circuit suffers greater power loss, with the increase of the load imposed on the fan motor.
With reference to four graphs shown in FIGS. 11A to 11D, the current fluctuation at the time of the switching of the stator coil (the point where the current sharply drops) is not prominent in the free-air state (FIG. 11A). However, as the fan motor shifts from the low-load state (FIG. 11B) to the high-load state (FIG. 11C), and further to the highest-load state (FIG. 11D), the current fluctuation range expands until the current waveform sharply rising in needle shape appears. It is apparent from FIGS. 11A to 11D that the height of the current waveform sharply rising in a needle shape becomes higher in the order of the low-load state, the high-load state, and the highest-load state.
As described above, with the existing fan motor control unit, the rotation speed of the fan motor drops and the air volume characteristic is degraded with the increase of the load imposed on the fan. In addition the circuit suffers greater power loss owing to the expansion of the current fluctuation range.